AMA Manual of Style - Stacy L. Christiansen, Cheryl Iverson 2020

Commonly Used Symbols
Mathematical Composition

Use mathematical symbols in running text only if they are presented with a numerical value within parentheses. These symbols can be used in figures, tables, and boxes with no explanatory footnotes. Some commonly used symbols are as follows:

Symbol	Description
>	greater than
≥	greater than or equal to
<	less than
≤	less than or equal to
±	plus or minus (This symbol should not be used to indicate variability around a central tendency, eg, “The control group had a mean [SD] value of 12 [7],” not “The control group had a mean of 12 ± 7.”)
	integral from value of a to value of b
	summation from a = 1 to a = 30
	product of a = 1 to a = 30
Δ	delta (change, difference between values)
f	function
≠	not equal to
≈	approximately equal to
∼	similar to (reserve for use in geometry and calculus, when it can be used in equations to mean “is distributed as”; use words in other cases where “approximately” is meant)
≅	congruent to
≡	defined as
∴	therefore
∞	infinity
!	factorial, eg, n! = n(n − 1) (n − 2) . . . 1

The following symbols are usually reserved for specific values:

π	pi (approximately 3.1416; do not confuse with uppercase Π)
e	base of the system of natural logarithms (approximately 2.7183; see 20.3.3, Logarithmic Expressions; in statistical equations, however, “e” represents the error term in a regression equation)
i	the square root of −1